The invention relates to a metal-ceramic substrate including at least one ceramic layer (2), which is provided with at least a first metallization (3) on a first surface side (2a) and with a second metallization (4) is provided on a second surface side (2b) lying opposite the first surface side (2a), wherein the at least one first metallization (3) is constituted by a foil or layer of copper or a copper alloy, wherein the first metallization (3) is bonded with the first surface side (2a) of the ceramic layer (2) with the aid of a “direct copper bonding” process and to a method for producing a metal-ceramic.
Metal-ceramic substrates in the form of printed circuit boards including an insulating layer of ceramic, and including at least one metallization bonded with a surface side of the insulating layer of ceramic and structured for the formation of strip conductors, contacts, contact and fastening regions, are known in various embodiments.
Of particular importance here is the cooling of such metal-ceramic substrates, which can take place for example by direct contact of the surface side of the ceramic lying opposite the structured metallization or of a metallization in thermally conductive connection with said ceramic with a cooling medium or a cooling body. Especially when use is made of a fluid or gaseous cooling medium, there are particular demands on the metal-ceramic substrate, for example with regard to resistance to corrosion and quality of the thermal conductivity. When such substrates are used in the motor vehicle sector, it is for example desirable to use the coolant circuit provided for cooling the vehicle also to cool the substrate.
The so-called “DCB process” (“direct copper bonding”), for example, is also known for bonding metal layers or sheets, preferably copper sheets or foils with one another and/or with ceramic or ceramic layers, and more precisely using metal or copper sheets or metal or copper foils which comprise on their surface sides a layer or a coating (“fusion layer”) comprising a chemical compound of the metal and a reactive gas, preferably oxygen. In this process described for example in US-PS 37 44 120 or in DE-PS 23 19 854, this layer or this coating (“fusion layer”) forms a eutectic with a melting temperature below the melting temperature of the metal (e.g. copper), so that, by placing the metal or copper foil onto the ceramic and by heating all the layers, the latter can be bonded to one another, and more precisely by fusion of the metal or copper essentially only in the region of the fusion layer or oxide layer. Such a DCB process comprises, for example, the following process steps:                oxidation of a copper foil in such a way that a uniform copper oxide layer results;        placing of the copper foil with the uniform copper oxide layer onto the ceramic layer;        heating of the composite to a process temperature between approx. 1025 to 1083° C., for example to approx. 1071° C.;        cooling to room temperature.        
Furthermore, the so-called active soldering process for bonding metal layers or metal foils forming s, in particular also copper layers or copper foils, with a ceramic material or a ceramic layer is known from publications DE 22 13 115 and EP-A-153 618. In this process, which is used especially also for producing metal-ceramic substrates, a bond is produced at a temperature between approx. 800-1000° C. between a metal foil, for example copper foil, a ceramic substrate, for example an aluminum nitride ceramic, using a hard solder, which also contains an active metal in addition to the main components such as copper, silver and gold. This active metal, which is for example at least one element of the group Hf, Ti, Zr, Nb, Ce, produces a bond between the hard solder and the ceramic through a chemical reaction, whilst the bond between the hard solder and the metal is a metallic hard soldered joint.
Proceeding from the aforementioned art, the problem underlying invention is to provide a metal-ceramic substrate as well as a method for its production, which is suitable for cooling by means of fluid or gaseous cooling media. The problem is solved by a metal-ceramic substrate and a method for its production.